Clarify that the manual is covered by a different licence.

This came up on IRC: a user thought that this page also covered the manual,
which is a reasonable assumption if you don't know that the manual and Web site
are different subprojects.

copyright.php

@@ -51,4 +51,11 @@ site_header("Copyright", array("current" => "footer"));
  <a href="http://php.net/">the PHP homepage</a>.
 </p>
 
+<h2>PHP Manual</h2>
+
+<p>
+ The PHP Manual is covered by a separate license, which is available
+ <a href="/manual/en/copyright.php">from within the manual itself</a>.
+</p>
+
 <?php site_footer();